Method and mold for casting thin wall cylinders



p 8, 1970 F. J. WEBBERE 3,527,285

METHQD AND MOLD FOR CASTING THIN WALL CYLINDERS Filed Nov. 7, 1967 IINVENTOR.

ATTORNEY United States Patent 3,527,285 METHOD AND MOLD FOR CASTING THIN WALL CYLINDERS Fred J. Webhere, Orchard Lake, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Nov. 7, 1967, Ser. No. 681,236 lint. Cl. B22d 13/02, 29/00 US. Cl. 164-114 3 Claims ABSTRACT OF THE DISCLOSURE Thin wall cylinders wherein the thickness of the wall is approximately equal to one-half of the diametric solidification contraction of the material of construction or smaller are centrifugally cast in a suitable mold and removed therefrom Without disassembly of the mold. An end plate is provided at one end only of the mold and a shallow weir is employed within the mold to retain the molten metal until it has solidified. The solidified article is removed over the weir through the open end of the mold.

This invention relates to the centrifugal casting of thin Wall cylinders. More specifically, it relates to a method of casting such cylinders to obtain drossand slag-free inner surfaces thereon and wherein a mold is employed from which the casting may be withdrawn without disassembly of the mold.

The production of articles in the configuration of a thin wall cylinder has conventionally been accomplished by one of two general methods. The cylinders either have been fabricated from wrought alloy sheet or tubing, or they have been produced by casting. In general, wrought alloys are more expensive than casting alloys because ingot cropping and billet conditioning contributes substantially to the manufactured cost of the former material. Therefore, casting sometimes offers a competitive advantage over other methods of production. However, thin wall cylindrical castings are not easily cast by conventional means which depend on coring techniques because of the high risk of misruns and laps. Moreover, thin wall cylinder castings are not readily prepared by centrifugal casting means because dross and other low-density impurities are found to accumulate on the inner surface of the casting. Thus, the inner surface of a centrifugal cast cylinder frequently requires extensive cleanup. In addition, the apparent need for mold disassembly to permit the casting removal usually results in relatively low production rates in centrifugal casting operations.

It is an object of the present invention to provide a method of centrifugally casting a thin wall cylinder which method utilizes relatively inexpensive casting alloys and eliminates the difficulties of removing the finished article from the mold.

It is a further object of the present invention to provide a method of centrifugally casting thin wall cylinders wherein the dross and other low-density impurities generated in the pouring operation are not deposited on the inner surface of the casting but are separated therefrom as an inherent part of the casting operation.

It is a further object of the present invention to provide a centrifugal casting mold suitable for casting thin wall cylinders which is characterized by one open end through which a casting having suitable wall dimensions may be removed without disassembly of the mold.

In accordance with the present invention, these and other objects are accomplished by first providing a generally cylindrical mold having an end plate, or other suitable dam or closure means, at only one of its ends.

The end plate is adapted to retain molten metal within the mold while the mold is being rotated about its longitudinal axis. However, it is preferred that the plate be annular in configuration so that molten metal may be introduced through the opening into the mold. The other end of the cylindrical mold is open. Intermediate the ends of the mold on the internal cylinder surface thereof is a small shallow land which serves as a weir. The height of the land above or inward from the internal cylindrical surface of the mold is preferably approximately equal to or less than one-half the diametric contraction of the material being cast upon solidification. The region of the mold between the land and the end plate is the space in which the cast article is formed. The weir and end plate cooperate to retain sufiicient molten metal within this region during rotation of the mold to form a cast cylinder of suitable thickness. Although there is substantial centrifugal force acting upon the molten metal it is somewhat sluggish in horizontal flow. Therefore, the wall thickness of the cast cylinder may be slightly greater than the height of the land. The portion of the mold cavity between the land and the open end preferably opens up to a larger unrestricted diameter. Excess metal and particularly low-density impurities flow over the weir into this unrestricted space. The quantity of metal poured and the temperature of the mold are suitably controlled so that no overflow molten metal or dross escape through the open end of the mold. Preferably the overflow material is frozen within the mold. Upon solidification of the cast material, contraction takes place and the cast article and impurities are readily withdrawn axially through the land opening and the open end, no disassembly of the mold being required.

These and other objects and advantages of the invention will become more apparent from a detailed de scription thereof reference being had to the attached drawings in which:

FIG. 1 shows an elevation view partly in section of a centrifugal mold in accordance with the invention and a centrifugal casting machine;

FIG. 2 is a perspective view partly broken away and in section of a centrifugal mold in accordance with the invention;

FIG. 3 shows a typical as-cast thin wall cylinder before removal of overflow metal; and

FIG. 4 shows a finished gas turbine engine combustor liner produced in part by the subject invention.

There are useful articles of commerce which in whole or in part are made up of a thin wall cylinder, wherein the thickness of the cylindrical wall is not substantially greater than one-half the diametric solidification contraction of the cast alloy. The gas turbine engine combustion liner 10 depicted in FIG. 4, is an example of such an article. I have discovered an economical method of producing such articles particularly when the wall thickness of the cast cylinder is in the range of 0.050 to 0.125". Under these circumstances a mold 12, such as is depicted in FIGS. 1 and 2, having an open end 14 and the other end 16 provided with a suitable end plate 18 may readily be adapted in the casting of such cylinders. Located on the inside of the mold 12 between ends 14 and 16 is a shallow land 20. Land 20 functions as a weir which cooperates with the end plate 18 to define therebetween a casting surface 22 on the inside of the mold 12. End plate 18 extends above or inwardly of surface 22 a distance sufficient to retain molten metal within mold 12 while the mold is rotating about its longitudinal axis. Preferably end plate 18 is annular as shown to admit a rotatable pouring trough 24 as shown in FIG. 1. The internal surface 26 of the mold between land 20 and open end 14 preferably is recessed slightly so that it is of greater diameter than surface 22 where- 3 by dross and other low-density impurities may flow over land 20, down slope 40 and out of the casting region 28 defined by end plate 18 mold surface 22 and land 20.

The height of land 20 above or inwardly of surface 22 is critical to the practice of the invention as it serves in large measure to define the wall thickness of the casting, particularly if it is intended to remove the cast article out through land 20 and through open end 14 without disassembly of the mold 12. Depending upon the outside diameter of the cylinder to be cast and the alloy being cast the maximum land height will change according to the total diametric solidification contraction and thermal contraction experienced during casting. The theoretical maximum land height is about one-half this diametric solidification contraction. The wall thickness of the cast cylinder, however, is dependent upon other casting conditions as will be described below, and usually is in the range of about 0.010" to 0.075 greater than the land height.

In accordance with the invention mold 12 is rotated about its longitudinal axis by a suitable variable speed roll drive assembly 30. Molten alloy 32 of suitable composition is prepared and added to rotatable pouring trough 24. The pouring trough 24 is driven by a variable speed drive 34. The mold 12 is rotated in one direction and the pouring trough 24 is then rotated in the opposite direction so that it empties into mold cavity 28. The molten alloy is of course forced against surface 22 by centrifugal force. End plate 18, or other suitable dam or closure means, retains the metal within the rotating mold at end 16. A portion of the molten metal is retained within space 28 by land 20 acting as a weir. However, some metal overflows the weir 20 carrying with it the dross and slag which might otherwise accumulate on the inside diameter of the casting. This excess material flows into the larger section 42 of the mold and solidifies quickly because of the large relatively cold mass of the mold. Preferably, the overflow never reaches end 14 of the mold. Upon solidification the cast material contracts so that its outside diameter is less than the inside diameter of land 20 and the casting together with the overflow metal is removed from the mold by drawing it through open end 14. In FIG. 3 is shown the as-cast article 36 with the solidified overflow metal and dross 38 attached thereto. When the overflow metal and dross 38 remains attached to the casting 36 after solidification it is readily removed by cutting therefrom.

A specific example will further illustrate the invention. It was desired to form a turbine engine combustion liner in accordance with the invention having dimensions of 7.6 OD by long with a wall thickness of 0.10". A cast iron mold was formed of the general design depicted in FIGS. 1 and 2. The mold had an overall length of about 20%". The outside diameter of the centrifugal mold was 10". Annular end plate 16 was approximately /2" thick and extended approximately 4" inwardly on or above casting surface 22. Casting surface 22 was tapered 0.05" at each side to permit easier removal of the casting therefrom through land 20 and open end 14 of the mold. Accordingly the diameter of surface 22 at end plate 18 was 7.755 and the diameter of the same surface at the land 20 was 7.855". The inside diameter of the land in this example was 7.755". The inside diameter of overflow surface 26 adjacent sloping portion 40 at the land was 8.30" and the inside diameter of surface 26 at open end 14 was 8.50". The length of surface 22 between end plate 18 and land 20 was 10.280

The combustion liner was cast using ACl HX ironnickel-chromium heat resistant alloy. This material has the following ACI specifications by weight: carbon 0.35- 0.75%, chromium -19%, nickel 64-68%, silicon 250% maximum, manganese 2.0% maximum, phosphorus 0.04% maximum, sulfur 0.04% maximum and the balance iron. A molten charge of about 9# of the materie a temperature of 2970 F.i10 F. was added to the pouring trough 24. The mold temperature was 600700 F. and the mold 12 was rotated counterclockwise about its horizontal axis at 525 rpm. Mold surface 22 had been coated with about 0.l# zirconium flour by employing an aqueous suspension thereof. Pouring trough 24 was rotated clockwise at 22 rpm. The molten alloy was observed to flow spirally covering the entire mold surface 22 and overflowing the shallow land 20 which acted as a weir. Excess metal flowing over the weir carried with it the dross and slag from the inside diameter of the casting. The mold was stopped almost immediately after pouring was completed because of the rapid solidification of the thin casting. The casting had contracted sufficiently to clear land 20 and was easily removed through the open end 14 of the mold. An air blast was observed to be helpful to hasten the cooling of the casting. The casting had overflow metal still attached as depicted in FIG. 3. This excess metal was readily removed with a friction-type band saw. In this example it is noted that the wall thickness of the casting was about 0.10" while the height of land 20 was only about 0.05".

The wall thickness of the casting is observed to be affected by the rate of rotation of the mold and the rate of rotation of the pouring trough as well as the height of the land or weir. In the above example the stated land height and rates of rotation of the pouring trough and mold, respectively, produced a casting wall thickness of 0.10". However, it was observed, while keeping the other mold and casting parameters constant, that an increase of in mold speed over the recommended 52 5 r.p.m. tended to decrease the wall thickness by approximately 20%. Conversely, decreasing the mold speed by 50% increased the casting wall thickness by about 20%.

It was also observed, when the mold rotational speed, land height and other parameters were held constant, that the trough roll over speed controlled to some extent the wall thickness of the casting. For example, if instead of a speed of 22 rpm. a faster trough rotation of 40 r.p.m. is employed the wall thickness is decreased to about 0.070 to 0.080. On the other hand if the trough rotation is reduced to 15 rpm. the wall thickness is increased to approximately 0.120.

Thus, it is to be understood that the thickness of the cast cylinder wall is likely to be somewhat greater than the height of the land or weir employed in the mold. The height of the land is for all practical purposes limited to one-half the diametric solidification contraction experienced with the casting alloy employed. Additional thermal contraction taking place in the casting after solidification is usually relied upon only to provide a small additional clearance for slipping the casting through the land. Of course, the absolute amount of solidification contraction is dependent in part upon the outside diameter of the casting. The increment in 'wall thickness contributed by the metal which, in the molten state, is actually above or inward of the inside surface of the land is dependent largely on the sluggishness of the metal and the rates of rotation of the mold and pouring trough as outlined above. When the land is designed to be quite shallow this latter increment may be as large as the increment contributed by metal held back by the weir. However, it normally is not large in absolute value. For example, in my experiences with cylinders of the above-described size and employing the iron-nickel-chromium alloy, the wall thicknesses obtained were in the range of 0.0100" to 0.075" greater than the land height.

While my invention has been described in terms of a specific embodiment thereof, it will be appreciated that other forms could readily be adopted by one skilled in the art and therefore the scope of my invention is to be considered limited only by the following claims.

I claim:

1. A method of centrifugally casting a thin wall cylinder comprising the steps of rotating a cylindrical mold body about its longitudinal axis, said mold body having a radially inwardly extending abutment member at only one end an annular land on the internal surface of said mold which is coaxial therewith and intermediate the ends thereof; pouring molten material into the portion of the mold between said abutment member and said land, the height of said land being not substantially greater than one-half the diametric solidification contraction of said material whereby excess molten material and lower density impurities flow over said land into the portion of said mold between said land and the open end thereof; cooling said molten material at least until it has solidified; stopping said mold; and removing the solidified casting through said land and the open end of said mold.

2. A method of centrifugally casting a thin wall cylin der comprising the steps of rotating a cylindrical mold body about its horizontal axis, said mold having one open end, closure means at the other end adapted to admit molten metal therethrough and then prevent said metal from flowing out of said end when said body is being rotated, and a weir on the inside cylindrical surface of said body coaxial therewith and intermediate the ends of said body; pouring molten metal into the portion of the mold between said closure means and said weir, the height of said weir being not substantially greater than the amount that said molten metal will shrink in the radial direction upon solidification and subsequent cooling, whereby excess molten metal and lower density impurities flow over said weir into the portion of said mold between said weir and the open end thereof; cooling said molten metal at least until it has solidified; stopping said mold and removing the solidified casting over said weir and through the open end of said mold.

3. A method of centrifugally casting a thin wall cylinder comprising rotating a cylindrical mold body about its longitudinal axis, said mold body having one open end, closure means at the other end adapted to admit molten metal therethrough and then prevent said metal from flowing out of said end when said body is being rotated, and a weir on the inside cylindrical surface of said body coaxial therewith and intermediate the ends of said body; pouring molten metal into the portion of the mold be tween said closure means and said weir, the height of said Weir being not substantially greater than the amount that said molten metal will shrink in the radial direction within said mold upon solidification and subsequent cooling, whereby excess molten metal and lower density impurities flow over said weir into the portion of said mold between said weir and the open end of said mold; cooling said molten metal at least unti it has solidified; stopping said mold; removing the solidified casting over said weir and through the open end of said mold and trimming the excess solidified metal and impurities from the end of the cylindrical casting.

References Cited UNITED STATES PATENTS 1,520,749 12/ 1924 Brownrigg 1641 14 1,678,633 7/1928 Burchartz 249-137 X FOREIGN PATENTS 157,950 2/ 1957 Sweden. 1,295,012 4/1962 France.

ROBERT D. BALDWIN, Primary Examiner US. Cl. X.R. 164-131, 299 

